The initial goal in this project was to express both protective antigens of dengue virus, i.e., the E glycoprotein and the NS1 nonstructural glycoprotein from cloned dengue cDNA using vaccinia virus as a vector. A recombinant vaccinia virus containing cloned DNA sequences coding for the three structural proteins and nonstructural proteins NS1 and NS2a of dengue type 4 virus was constructed. Infection of CV-1 cells with this recombinant virus produced dengue structural proteins as well as the nonstructural protein NS1. These proteins were precipitated by specific antisera and exhibited the same molecular size and glycosylation patterns as authentic dengue viral proteins. Infection of cotton rats with the recombinant virus induced NS1 antibodies in one of 11 animals. However, an immune response to either the pre M or the E glycoproteins was not detected. A low level of expression of dengue genes by the vaccinia virus recombinant was probably the explanation for the limited serologic response to these dengue antigens.